A new radiologic method (direct photon absorptiometry) appears useful for assessment of growth and development. Bone mineral content and soft-tissue composition (lean vs fat) can be measured with high precision and accuracy (less than 2 percent error) by radionuclide (125-I and 241-Am) scanning. This enables objective detection of small differences among individuals, or in the same individual over time. The procedure requires only a few minutes of subject time, the equipment is portable and relatively easy to use, no substances are ingested or injected, no biopsies or specimen are needed, the procedure is innocuous, and the radiation dose to the subject is minimal (10 mrem) and confined to a small area on the extremities. We will conduct (a) methodological studies to validate the absorptiometric method on children and to optimize techniques for field studies, (b) clinical studies to provide information on medical applications (diagnosis, and therapyevaluation) in children with growth disorders and selected disease states, and (c) field studies of compositional growth and development in children aged 5 to 20 years in order to define the patterns for different socio-economic and/or ethnic groups. This three phase project will be carried out over a three year period. These studies could open up new areas of biomedical research and novel clinical applications dependent on direct and simple determination of soft-tissue composition, hydration, and bone mineral content.